This study will test the following hypotheses about the somatotopic organization of the hindlimb region of the cat dorsal horn: (1) the skin representation at each rostrocaudal (RC) level of the dorsal horn is determined but the miniature dermatomes of dorsal rootlets ("dermatomelets") at that level; (2) peripheral innervation density determines map scale (number of dorsal horn cells devoted to a unit area of skin); (3) the divergence of connections for a given afferent type. and (4) the convergence of afferent input upon a dorsal horn cell are constant throughout the map; (5) the distributions of afferent terminals and cell dendrites are determined by (1) - (4); (6) dorsal horn cell receptive fields (RFs) are determined by (1) - (5). RFs of axons in dorsal rootlets will be sampled using teased-fiber recording. RFs of dorsal horn cells at the same RC levels will be sampled using extracellular recording. Light touch innervation fields (IFs) of peripheral cutaneous nerves will be mapped electrophysiologically and the A fibers in the nerves will be counted. The probabilities of monosynaptic connections from afferents to dorsal horn cells will be determined with two methods: (a) single axons will be stimulated electrically throughout cells' RFs; and (b) single dorsal root ganglion cells will be stimulated intracellularly. Postsynaptic single unit response times minus arrival times of afferent action potentials will indicate synaptic delay. A model dorsal horn map with high resolution will be obtained from extracellular recording data. Primary afferent terminal distributions and dorsal horn cell dendrite distributions will be determined using intracellular injection of neurobiotin. These data will be used to construct a first-approximation model of the dorsal horn map of the skin, in three steps: (1) delineation of map gradients from dermatomelet data; (2) prediction of axonal arbors and dendritic tree geometries from the gradients; and (3) simulation of RFs of dorsal horn cells from calculations of probability of contact, based on axonal and dendritic distributions.